1. Field of the Invention
This invention relates to the two-component polyurethane coating compositions containing polyisocyanates and certain aliphatic polyester polyols as the binder and their use for coating flexible substrates, more particularly plastic moldings in automobile construction.
2. Description of the Prior Art
It has long been known that inflexible substrates, such as metals or wood, can be coated with two-component polyurethane coating compositions based on hydroxy-functional polyesters, polyethers or polyacrylates and organic polyisocyanates. The resulting coatings are distinguished in particular by excellent hardness, by very good adhesion and by high weather resistance. The chemical components of these coating compositions and coatings are described inter alia in xe2x80x9cLackkunstharzexe2x80x9d, Hans Wagner/Hans Friedrich Sarx, Carl Hanser Verlag, Mxc3xcnchen, pages 153 to 173, 1971.
However, known two-component polyurethane coating compositions often give highly crosslinked lacquer coatings in which the elasticity and resistance to yellowing, particularly on exposure to short-wave UV light, often fail to satisfy the requirements that coatings for flexible substrates are expected to meet.
In automobile construction in particular, flexible plastic parts are being used to an increasing extent to improve safety. In some countries, for example, vehicles have to be fitted with fenders which are capable of withstanding impact against a fixed barrier without damage at a certain speed (for example 5 mph). Functional components, such as headlamps, indicator lamps, doors and hoods, must also remain operatively intact. In many countries the effect of these and other more exacting requirements has been that metal fenders are no longer used in automobile construction. They have been replaced by complete front and rear sections of high-flexibility materials that meet the need for damage-free restorability of the fender after impact (avoidance of damage in minor accidents).
High-flexibility plastics which have proved to be suitable for this particular application include semirigid elastomeric polyurethanes (such as those prepared from the Bayflex resins available from Bayer AG, Leverkusen). The elastomeric polyurethanes are produced from two-component polyurethane-forming mixtures by the reaction injection molding process in closed molds, optionally with foaming. Also suitable are thermoplastic polyurethanes (for example the Desmopan resins available from Bayer AG or the Texin resins available from Miles Inc., Pittsburgh, Pa.), which are processed by injection molding, and also various types of rubbers.
In the field of automotive construction, plastics based on these man-made materials which have been known for some years are relatively large and, hence, play a significant part in determining the appearance of the vehicle. For these reasons, the plastics used have to be coated. Another factor to be taken into account is that the surfaces of the plastics are degraded on weathering and, thus, have to be protected against the effects of weathering.
Unfortunately, elastic coatings are also required for plastic parts of low elasticity in order to prevent mechanical damage to the parts in question. For example, rigid but tough thermoplastics have to be coated with highly elastic, extremely resistant coating compositions to prevent cracks from developing in the coating in the event of mechanical damage or under the effect of other external influences and then spreading into the compact plastic.
It is possible using known two-component systems to produce highly flexible coating compositions which are totally adequate in terms of elasticity and flexibility at low temperatures. Such coating compositions use as the key polyol component certain polyhydroxyl polyesters which are predominantly synthesized from aliphatic diols and which also have a hydroxyl functionality of slightly above 2. The disadvantage of two-component polyurethane coating compositions based on these polyester diols and typical lacquer polyisocyanates lies in the fact that the coatings show inadequate water resistance and chalking resistance so that the gloss retention of coatings produced from these lacquer systems is totally inadequate. A particular disadvantage of these coating compositions is their tendency to yellow upon exposure to short-wave UV light. Another major disadvantage is that corresponding coatings lack tar stain resistance. This applies in particular to the systems described in DE-OS 3,421,122 which otherwise largely satisfy the above-mentioned requirements that elastic polymer coatings are expected to meet.
A significant improvement in the susceptibility of highly elastic polyurethane coatings to yellow upon exposure to short-wave UV light can be obtained with the polyester polyols described in EP-A 0,318,800. In addition, improvements in the tar stain resistance of the resulting polyurethane coatings can also be obtained with these polyester polyols, although these improvements are still not sufficient to meet present-day requirements.
Accordingly, an object of the present invention is to provide a new two-component polyurethane coating composition which satisfies the preceding requirements and which would be optimally suitable in particular for the coating of elastic plastic parts. The new coating composition should satisfy in particular the following requirements:
1. Film surfaces free from yellowing upon exposure to short-wave UV light: The coating should withstand exposure to short-wave UV light (wavelength  less than 400 nm) for at least 200 hours without yellowing.
2. Low-temperature elasticity: The film should not crack under impact at the above-mentioned speeds, even at xe2x88x9215xc2x0 C. The mechanical properties of the plastic should not be affected by the coating.
3. Resistance to tar stains: The coating should withstand exposure to a special tar-containing test solution without damage.
4. Good gloss retention:
The coating on the plastic parts should be highly durable. It should not degrade any more quickly under the effect of weathering than coatings on bodywork.
5. No post-embrittlement on weathering: The coating should not undergo post-embrittlement, even after prolonged weathering, i.e., it should retain its elasticity at low temperatures, even during weathering.
6. Low drying temperature: The heat resistance of large plastic parts of the type in question is limited. For this reason, the necessary drying temperatures and times should be as low and as short as possible both for this reason and for reasons of saving energy.
7. Reparability: The drying conditions of such a system should allow not only for initial (OEM) coating but also for repair coating at 80xc2x0 C. or room temperature.
It has now surprisingly been found that these objects can be achieved and, in particular, coatings resistant to tar stains can be obtained by using the polyester polyols according to the invention, based on the starting materials described in more detail hereinafter, as the polyhydroxyl component or as the essential part of the polyhydroxyl component in two-component polyurethane coating compositions.
The present invention relates to two-component polyurethane coating compositions, which are suitable for the production of flexible coatings on plastic moldings, wherein the binders contain one or more lacquer polyisocyanates and a polyol component containing one or more polyester polyols which have a hydroxyl number of 80 to 250 and are the reaction products of a) 52 to 60 mol-% of a polyol component containing
i) 5 to 50 mol-% of one or more dihydric aliphatic alcohols having at least 2 carbon atoms other than neopentyl glycol,
ii) 5 to 40 mol-% of one or more at least trihydric aliphatic alcohols having at least 3 carbon atoms,
iii) 0 to 9 mol-% of one or more diols containing cycloaliphatic units and
iv) 20 to 90 mol-% of neopentyl glycol, and b) 40 to 48 mol-% of a dicarboxylic acid component containing
v) 0 to 49.9% by weight of one or more aliphatic saturated dicarboxylic acids or dicarboxylic anhydrides containing at least two carbon atoms and
vi) 50.1 to 100 mol-% of one or more aliphatic unsaturated dicarboxylic acids. or dicarboxylic anhydrides containing at least 4 carbon atoms,
wherein the percentages set forth in a) and b) add up to 100.
The present invention also relates to the use of these coating compositions for coating flexible substrates, more particularly plastic moldings in automobile construction.